Stainless steel powder intermixed with boron nitride powder for enhanced machinability of sintered powder metal parts

ABSTRACT

An elemental powder or powder of compounds is disclosed consisting essentially of the primary metals chromium, nickel and molybdenum for blending with an atomized, prealloyed, stainless steel powder and followed by pressing the powder into a part for sintering includes an additive incorporated into the matrix of the stainless steel powder that enhances the machinability of the part and serves as a chip breaker. Among the candidate additives are hexagonal boron nitride, a powder with lubricating qualities, Monel Metal (Ni—Cu), cupro-nickel (Cu—Ni), and powders of electrical resistance alloys such as 80-20 alloy (80Ni—20Cr), 70-30 NiCr, Ni—Cr—Fe—Si alloys, and molybdenum disilicide (MoSi 2 ).

FIELD OF THE INVENTION

[0001] The present invention pertains to a composition including stainless steel powder intermixed with an additive for enhancing the machinability of the sintered stainless steel powder metal parts, and, more particularly, pertains to the incorporation of boron nitride with stainless steel powder to enhance the machinability of the formed sintered stainless steel powder metal parts.

BACKGROUND OF THE INVENTION

[0002] With reference to prior U.S. Pat. Nos. 3,980,444 (Reen), 4,014,680 (Reen), and 4,032,336 (Reen), these patents disclose a sintered stainless steel having an overall density of at least 95% of full density and a morphology comprised of regions of sintered austentitic stainless steel and regions of solidified liquid phase. Sintering is a process of forming metallic powder into a homogeneous mass without melting. In general, the powder is first pressed into the desired part and then heated in an oven. The above-referenced patents disclose a sintered stainless steel having a composition consisting of essentially of, by weight, up to 0.05% carbon, 22% to 26% chromium, 10% to 24% nickel, 2.7% to 5% molybdenum, 0.1% to 1% boron, up to 2.0% manganese, up to 2.0% silicon, and the balance iron and residuals. U.S. Pat. No. 4,014,680 discloses this composition as a prealloyed stainless steel powder for use in the sintering process. U.S. Pat. No. 3,980,444 discloses the sintered stainless steel that is produced from the afore-described composition, and U.S. Pat. No. 4,032,336 discloses the method of making the sintered stainless steel having this composition by using the stainless steel powder described in U.S. Pat. No. 4,014,680. All of the above patents further disclose that the use of the composition increases the density of the sintered stainless steel, resulting in an increased resistance to corrosive attack by the chloride ion.

[0003] Solid stainless steel parts produced by the composition and process described in the above-cited patents achieve a high density by forming a liquid phase during the sintering process, and that is known as liquid phase sintering. However, after the solid stainless steel part has been liquid phase sintered, there are circumstances in which it is necessary to further machine the part. For example, the part may need machining to obtain a specific dimensional tolerance, or the part may require threading—something that cannot be accomplished during the pressing process.

[0004] Yet, if the microstructure of the parts has little porosity, the part is thus rendered difficult to machine. In this respect the sintered stainless steel parts produced according to the techniques set forth in the three above-cited patents are similar to austenitic stainless steels in wrought form, such as bar stock that is produced by conventional steel making processes that use melting.

[0005] Machinability shortcomings with wrought steel have previously been alleviated by introducing specific particles, both metallic and non-metallic, throughout the stainless steel matrix. The particles introduced have included lead, selenium sulfide, and manganese sulfide. The production and use of wrought stainless steel containing lead and selenium sulfide has been declining due to toxicity concerns engendered from the melting process.

SUMMARY OF THE INVENTION

[0006] The present invention comprehends a non-sulfur bearing additive material that is incorporated into a stainless steel powder for increasing the machinability of the formed sintered stainless steel powdered metal part but without diminishing the corrosion resistance of the powdered metal part. Specifically, the present invention comprehends the addition of hexagonal boron nitride to a base metal composition for increasing the machinability of the composition. More fully, the present invention pertains to the incorporation of elemental powder and/or powder of compounds derived from such primary metals as chromium, nickel, and molybdenum with the atomized, prealloyed, stainless steel powder. This is followed by pressing the powder into a metal part and then subjecting that part to sintering. The additive would serve as a chip breaker, and a primary candidate is boron nitride (of various grades), a powder with enhanced lubricating qualities. Moreover, as an additive for all types of stainless steel powder, boron nitride may be particularly adapted for grade 1-999 type powdered metal stainless steel parts. In addition, other possible candidate additives are powders of Monel Metal (Ni—Cu), cupro -nickel (Cu—Ni), and powders of electrical resistance alloys such as 80-20 alloy (80Ni—20Cr), 70Ni—30Cr, Ni—Cr—Fe—Si alloys, and molybdenum disilicide (MoSi₂).

[0007] It is an objective of the present invention to incorporate an additive into the matrix of a high density austentitic stainless steel part produced from an atomized prealloyed powder.

[0008] It is another objective of the present invention to incorporate an additive into the matrix of a stainless steel powdered metal part for enhancing the machinability of the powder metal part without diminishing the corrosion resistance of the powder metal part to the chloride ion.

[0009] It is yet another objective of the present invention to produce machinable austentitic stainless steel parts of varying densities without the use of manganese sulfide particles as the additive.

[0010] Still another objective of the present invention is to use boron nitride of various grades as the additive for incorporation with the prealloyed and atomized austentitic stainless steel powder to improve the machinability of stainless steel powdered metal parts.

[0011] These and other objects, features, and advantages will become apparent to those skilled in the art upon a perusal of the following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a sectioned side elevational view of a hexagonal ring; and

[0013]FIG. 2 is a front elevational view of the hexagonal ring first shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0014] With reference to Tables I-XII and FIGS. 1 and 2, using the powder with the processing methods described in U.S. Pat. Nos. 3,980,444, 4,014,680 and 4,032,336, sintered stainless steel parts are produced within the composition range set forth in these patents. U.S. Pat. No. 3,980,444 sets forth the manner of producing sintered parts from a powder having a composition set forth in U.S. Pat. No. 4,014,680, and with the manufacturing process set forth in U.S. Pat. No. 4,032,336. The composition range set forth in U.S. Pat. No. 4,014,680 was designed so that a liquid phase would form in the part during sintering and produce an essentially full density part. However, sintered parts made in accordance with these patents are difficult to machine. It has historically been shown that the presence of manganese sulfide particles in a stainless steel matrix enhances the machinability of the part. Moreover, it has also been shown that the presence of manganese sulfide particles lowers the resistance of the part to the chloride ion to an undesirable level. With the realization that the manganese sulfide particles in the stainless steel matrix serve as chip breakers during the machining process, it was determined to use elemental powder and/or powder of compounds of the primary metals of stainless steel, chromium, nickel, and molybdenum as chip breaker additives. In addition, other compounds suggested themselves as candidate additive materials such as boron nitride, Monel Metal, cupro-nickel, nickel-chromium, and molybdenum disilicide

[0015] Because the above additives have different densities, it is difficult to make a proper evaluation of them by comparison with additives having the same weight as the stainless steel powder. As shown in Table I, a more valid comparison is made by adding equal volumes of the additives to the stainless steel powder. In Tables II-IV, the properties of Monel Metal, hexagonal boron nitride powder, and molybdenum powder are set forth while Table V shows the properties of the stainless steel powder.

[0016] Thus, equivalent volumes of powders of six of the additives were blended with the stainless steel powder, and the blended powders were then processed into sintered parts. Further, the densities, machinability, and corrosion resistance of the sintered parts to immersion in six per cent solution of sodium chloride were determined with the results shown in Table VI. The sintered parts used for the machinability and corrosion tests are hereinafter further described.

[0017] In general, the following results as set forth in Table VI were obtained. 1.) The full density of the parts was achieved with only two additives to the grade I-999 powder -molybdenum and hexagonal boron nitride; 2.) The addition of molybdenum to the powder had a detrimental effect on the machinability of the powder; 3.) The addition of hexagonal boron nitride and Monel Metal to the grade I-999 powder had a beneficial effect on the machinability of the parts in so far as only one cutting edge was needed to machine about the same number of parts as needed for parts with no additive that required three cutting edges; 4.) Parts with machinability additives of Monel Metal, MoSi₂, CrSi₂, and 80Ni-20Cr did not achieve a sufficiently high percentage of full density to permit testing; and 5.) Parts with Monel Metal and 80Ni-20Cr additives rusted in the immersion test. As a result of the boron nitride providing an increase in the machinability for sintered stainless steel and passing the salt immersion corrosion test, boron nitride served as the basis for all further testing and evaluation as an additive.

[0018] In evaluating boron nitride, it must be noted that hexagonal boron nitride has several sources that vary in degrees of purity, particle size, and apparent density. Boron nitride for evaluation was selected from two sources, Cerac and Carborundum, and the test data is shown in Table VII. Table VIII illustrates the various properties of the stainless steel designated as grade I-999 that was used to make blends 21-1 to 21-7 with the boron nitride powder.

[0019] Table IX illustrates the calculations for converting various weight percentages of sulfur (that produces manganese sulfide) in a wrought stainless steel alloy to an equivalent weight per cent of boron nitride in order to produce the same volume as the manganese sulfide.

[0020] Table X illustrates the various blends of grade I-999 with 0.20 and up to 0.61 weight per cent hexagonal boron nitride. Boron nitride of 0.20 and 0.61 weight per cent will produce the same volume of particles as the manganese sulfide formed with 0.13 and 0.40 weight percent sulfur.

[0021] In Table XI the results of certain machinability tests are shown, and the tests indicate that: 1.) More parts of grade I-999 that contain hexagonal boron nitride, regardless of source, amount and particle size attained better machinability than parts with no hexagonal boron nitride; 2.) A fine particle size of hexagonal boron nitride of—325 mesh produced more machinable parts than the larger particle size; and 3.) although almost any amount of fine hexagonal boron nitride improves machinability of grade I-999, it appears that an optimum amount is 0.38 weight per cent.

[0022] Table XII illustrates the corrosion resistance evaluation of sintered parts according to two test methods of the American Society of Testing Materials standard test methods, A 262—Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels, Practice A (screen test) and G48—Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution, Methods A and B at 22 Degrees Centigrade.

[0023] In the screening test of ASTM A—262, Practice A, two metallographic samples of each sintered part were tested using a 10% oxalic acid solution as etchant. Based on these etched microstructures, the sintered parts were determined to be essentially free of susceptibility to intergranular attack associated with chromium carbide precipitates.

[0024] In Method A for the ferric chloride pitting test all the samples were minimally susceptible to pitting corrosion as measured by this test. In Method B, the ferric chloride crevice test, both test specimens of grade I-999 plus 0.38 boron nitride appeared to be minimally susceptible to crevice corrosion as measured by this test. Furthermore, samples of grade 1-999 plus 0.61% boron nitride were somewhat more susceptible to crevice corrosion as measured by this test.

[0025] With reference to Tables VI and X, processing of the various additives was as follows: 10 to 20 pound blends of atomized prealloyed stainless steel powder and additives were prepared; for pressing, a hexagonal ring, as shown in FIGS. 1 and 2, was double action pressed to a green density of approximately 6.65 grams per cubic centimeter; with regard to sintering, all the parts were presintered at 2200 degrees Fahrenheit in a continuous belt furnace and then vacuum sintered at 2265 degrees Fahrenheit.

[0026] The powdered metal parts used in the above processes were machined on a South Bend lathe, Model 1218 Magnatum having the following working conditions. The cutting tool was an Iscar Grade IC9025, positive seven degree clearance, 80 degree rhombic insert, ANSI designation CCMT 2-1-14. This tool is a cemented tungsten carbide insert that has a cobalt enriched ISCAR Grade IC-24S substrate that has been chemical vapor deposition coated with TiC/TiCN/TiN. The cutting lubricant was a 10 per cent solution of Valentite VNT 920 having properties described in U.S. Pat. No. 5,936,170. The sintered parts were machined in two passes with the first pass having a feed of 0.0060 inches/revolution (ipr), a speed of 1500 revolutions per minute (rpm) at 220 surface feet per minute (sfm), and a depth of cut of 0.009 inches (doc). For the second pass, the feed was 0.0049 (ipr), the speed was 1500 rpms at 220 sfm, and the depth of cut was 0.0113 inches.

[0027] The parts for each blend were machined separately with a single edge of the cemented carbide insert until the insert no longer produced a satisfactory surface finish and/or size consistency. The carbide insert was then rotated and another cutting edge was used, and the parts machined and the number of cutting edges used for each blend are set forth in Tables VI and XI. It is from this data that the parts machined per edge was calculated.

[0028] The foregoing description discloses and describes a preferred embodiment for the invention, and those skilled in the art will understand that other variations and modifications may be possible and practicable, and still come within the ambit of the appended claims. TABLE I ADDITIVES TO GRADE I-999 FOR MACHINABILITY In wrought AISI Type 303 stainless steel, the aim of its sulfur content is 0.25 percent. It is assumed that all of the sulfur reacts with the manganese in the alloy to form manganese sulfide. The atomic weight of sulfur is 32.07 and manganese is 54.93. For a weight of 0.25 grams of Mn, 0.68 grams of MnS is produced. Manganese sulfide has a density of 3.99 g/cc. 0.68 grams of MnS has a volume of 0.170 cubic centimeters. The weight of the additives may be calculated by multiplying this value by the additive density to obtain equivalent volumes of 0.25 grams of MnS. Weight percent of experimental additives to obtain equivalent volume percentage produced by 0.25 weight percentage of sulfur density Additive g/cc Weight Percent Molybdenum 10.2 1.74 Monel Metal 8.0 1.36 Boron Nitride 2.25 0.38 Molybdenum Disilicide 6.31 1.07 Chromium Disilicide 4.29 0.73 80 Ni - 20 Cr 8.40 1.42

[0029]

TABLE V CHARACTERISTICS OF THE ATOMIZED PRE-ALLOYED METAL POWDER (AMETEK LOT 1016600) USED IN THE BLENDS CHEMICAL COMPOSITION, WEIGHT PERCENT Carbon 0.035 Manganese 0.06 Phosphorus 0.022 Sulfur 0.014 Silicon 0.68 Chromium 22.47 Nickel 18.29 Molybdenum 3.33 Boron 0.34 Oxygen 0.47 Nitrogen 0.037 Iron Balance Particle Size Distribution (US Standard Mesh Size) in Percent +100 4.0 +120 5.4 +140 7.0 +200 17.0 +270 20.8 +325 9.8 −325 36.0 Apparent Density 2.96 grams/cubic centimeter Flow Rate 31 seconds/50 grams

[0030] TABLE VI CALCULATED SINTERED DENSITIES OF GRADE I-999 PARTS CONTAINING ADDITIVES ASSUMING THE FULL DENSITY OF GRADE I-999 IS 7.85 g/cc ADDITIVE SOURCE LOT NO. DENSITY-g/cc % ADDITIVE % GRADE I-999 Mo Cyprus  1908 10.2 1.74 98.26 Monel Metal Ametek 513096 8.00 1.36 98.64 BN Cerac  4989 2.25 0.38 99.62 MoSi₂ Cerac 187570-A-(1,2) 6.31 1.07 98.93 CrSi₂ Cerac 55948-A-1 4.30 0.73 99.27 NiCr Cerac X200783 8.40 1.42 98.58 Using the Law of Mixtures, the densities of the sintered blends were calculated. These values were compared with the actual densities as shown in the table below. The sintered parts were immersed in a six percent salt solution for 48 hours to see if they would rust. CALCULATED PART DENSITY ADDITIVE DENSITY - g/cc RANGE - g/cc RUST/NO RUST Mo 7.88 7.86-7.89 NO RUST Monel Metal 7.85 7.52-7.58 RUST BN 7.78 7.83-7.86 NO RUST MoSi₂ 7.83 7.55-7.79 NO RUST CrSi₂ 7.80 7.67-7.76 NO RUST NiCr 7.86 7.52-7.60 RUST MACHINABILITY TESTS CUTTING PARTS PARTS EDGES MACHINED ADDITIVE MACHINED USED PER EDGE Mo 125 4 31 Monel Metal 110 1 110 BN 110 1 110 MoSi 116 4 29 CrSi₂ 53 4 13 NiCr 92 2 46 None 339 3 113

[0031] TABLE VII TYPICAL PROPERTIES OF CARBORUNDUM HEXAGONAL BORON NITRIDE (BN) POWDER USED IN THE BLENDS Apparent Surface Tap Density BN B₂O₃ O₂ US Mesh Area Density Grade Class % % % 90% min. m²/g g/cm² HPC-40 high 99.0 0.1 0.3 −40/+140  4 0.9 HPF-325 high 99.5 0.3 0.3 −325 10 0.8 Density: 2.27 g/cm³ TYPICAL PROPERTIES OF CERAC HEXAGONAL BORON NITRIDE (BN) POWDER USED IN THE BLENDS Item No B-1083 in Cerac Incorporated catalog Particle size −80 mesh average or less (from catalog) Typical Purity: 97.5% (from catalog) Primary Impurities (from data sheet) 3 ppm zinc 32 ppm silicon

[0032] TABLE VIII CHARACTERISTICS OF THE ATOMIZED PREALLOYED METAL POWDER (AMETEK LOT 052100) USED IN THE BLENDS CHEMICAL COMPOSITION, WEIGHT PERCENT Carbon 0.027 Manganese 0.11 Phosphorus 0.02 Sulfur 0.016 Silicon 0.77 Chromium 23.15 Nickel 17.73 Molybdenum 3.55 Boron 0.29 Oxygen 0.52 Nitrogen 0.013 Iron Balance Particle Size Distribution (US Standard Mesh Size) in Percent +100 3.3 +120 3.6 +140 7.6 +200 18.3 +270 21.0 +325 10.3 −325 35.9 Apparent Density 2.96 grams/cubic centimeter Flow Rate 26 seconds/50 grams

[0033] TABLE IX CALCULATIONS FOR CONVERTING WEIGHT PERCENT OF MANGANESE SULFIDE TO WEIGHT PERCENT OF BORON NITRIDE TO ACHIEVE EQUAL VOLUMES IN A STAINLESS STEEL MATRIX Sulfur content in wrought AISI Type 303 stainless steel: 0.15 percent minimum Normal melt range: 0.20-0.30, aim 0.25 percent sulfur Densities MnS: 3.99 g/cc BN: 2.25 g/cc Equations S + Mn→ MnS for weight of MnS in grams Weight of MnS ÷ 3.99 g/cc for volume of MnS in cc Volume of MnS × 2.25 g/cc for weight of BN in grams S-% MnS Produced-g MnS Produced-cc Equivalent BN-g 0.10 0.27 0.068 0.15 0.15 0.41 0.102 0.23 0.20 0.54 0.135 0.30 0.25 0.68 0.170 0.38 0.30 0.82 0.204 0.46 0.35 0.95 0.237 0.54 0.40 1.08 0.272 0.61

[0034] TABLE X BLENDS OF I-999 POWDER AND BORON NITRIDE POWDER Blend BN* Apparent Mesh BN Added Number Source Density Size Percent 21-3A A —  −80 0.38 21-3B C High  −40 0.38 21-3C C High −325 0.20 21-3D C High −325 0.30 21-3E C High −325 0.38 21-3F C High −325 0.46 21-3G C High −325 0.61 21-3H None

[0035] TABLE XI MACHINABILITY TESTS CUTTING PERCENT PARTS EDGES PARTS BLEND ADDITIVE MACHINED USED PER EDGE 21-3A 0.38 CERAC 426 2 213 21-3B 0.38 HPC-40 473 3 158 21-3C 0.20 HFF-325 476 4 110 21-3D 0.30 HPF-325 498 6  83 21-3E 0.38 HPF-325 496 2 248 21-3F 0.46 HPF-325 485 5  97 21-3G 0.61 HPF-325 492 3 164 21-3H NONE 485 6  81

[0036] TABLE XII CORROSION RESISTANCE TESTS - ASTM G-48 All the parts with hexagonal boron nitride were made with Carborundum Grade HPF ASTM G48 - Method A - Ferric Chloride Pitting Test at 22 +/− 2° C. Weight Change Grade I-999 −0.0103  Grade I-999 + 0.38% BN, Test 1 −0.00009 Grade I-999 + 0.38% BN, Test 2 −0.00005 Grade I-999 + 0.61% BN, Test 1 −0.00003 Grade I-999 + 0.61% BN, Test 2 −0.00005 ASTM G48 - Method B - Ferric Chloride Crevice Test at 22 +/− 2° C. Weight Change - Testing Time - Hrs g/cm² Grade I-999 72 −0.00001 Grade I-999 72 −0.00023 Grade I-999 72 −0.00002 Grade I-999 + 0.38 BN, Test 1 72 −0.00016 Grade I-999 + 0.38 BN, Test 2 72 −0.00926 Grade I-999 + 0.61 BN, Test I 72 −0.00809 Grade I-999 + 0.61 BN, Test 2 72 −0.00121 

We claim:
 1. A blend consisting of pre-alloyed stainless steel powder consisting essentially of, by weight, up to 0.05% carbon, 22% to 26% chromium, 10% to 24% nickel, 2.7% to 5% molybdenum, 0.1% to 1% boron, up to 2.0% manganese, up to 2.0% silicon, balance iron and residuals, together with boron nitride particles added thereto essentially, by weight, from 0.25% of the pre-alloyed stainless steel powder upward.
 2. A sintered stainless steel having excellent resistance to corrosive attack by the chloride ion and improved machinability, said steel having an overall composition consisting essentially of, by weight, up to 0.05% carbon, 22% to 26% chromium, 10% to 24% nickel, 2.7% 5% molybdenum, 0.1% to 1.0% boron, up to 2.0% manganese, up to 2.0% silicon, up to 0.4% boron nitride, balance iron and residuals. 